Four prominent admen are forming a new agency in San Francisco, the 18th shop with roots in Hal Riney & Partners. Former Riney senior executive John Yost; Steve Stone, senior VP, and co-creative director on Riney's Saturn Motor Corp.'s account; Mark Bardon, director of account planning at Riney; and Bob Kerstetter, a Goodby
, Silverstein & Partners associate creative director and copywriter, are developing final plans for the agency. Mr. Yost, 46, held top Riney management positions for nine years, including director of corporate development, before leaving at the end of the 1995. Mr. Yost and other members of the new shop did not return phone calls.